From my understanding, software tech companies were the first companies to have near $0 marginal cost customers. Generally, I don't think people refer to GE, Boeing, any energy company or any auto company as "tech" companies. These companies develop significant technology.

Tech seems to be branding for modernism, software, and rapid low-cost expansion. Lots of businesses are scalable in some sense. There are a lot of MacDonalds out there. I would say the primary difference is adding a new customer costs virtually nothing. This seems to be a relatively unique property of software companies.

Yup. It’s a metonym that’s getting at the idea of “bits, not atoms.”

Plenty of companies use extremely impressive tech to manipulate atoms, but, by convention, we call the ones that manipulate bits “tech companies.”

It's short for "Information Technology", which is what these companies used to be called and illuminates the whole "scale" factor that has come to dominate discussions about what distinguishes them.